A “Big Bird” descendant from the now-infamous Española Cactus-Finch and Medium Ground-Finch that met on Daphne Major island.Photo:Reproduced with permission from Princeton University Press, which first published the image in 40 Years of Evolution (P. R. Grant & B. R. Grant, 2014)

Thirty-six years ago, an unfamiliar schnoz popped up on Daphne Major.

Over the past four decades, Rosemary and Peter Grant have identified and banded every finch that lives on this tiny, dog-bowl-shaped isle in the heart of the Galapagos archipelago. So, in 1981, when a member of their research team noticed a young male with an imposing beak and a strange song, the Princeton University scientists instantly knew it was out of place. A blood sample confirmed their hunch: The bird was an Española Cactus-Finch, and it had crossed miles of ocean to mingle with the local Medium Ground-Finches.

And that’s how the latest, most bizarre case of finch evolution began: with a wanderer and then a bang. The exotic male from Española island quickly mated with a native female, giving rise to a group of stocky-beaked finches that biologists dubbed the “Big Bird” lineage. After just three generations, the Big Birds started pairing up with hybrids exclusively, and are now on their way to forming a new species, a study recently published in Science reveals.

The culprit behind this evolutionary shift is the same big-beak bias that’s been documented time and time again in the Galapagos, Sangeet Lamichhaney, the lead author of the Science paper and former genomics student at Sweden’s Uppsala University, says. The descendants of the Española Cactus-Finch are well endowed bill-wise; by interbreeding, they can ensure that their offspring have an edge when it comes to foraging for fruits and seeds. Every year during the study, hybrids on Daphne Major were born with deeper beaks and had higher rates of survival; in 2009, for example, all 19 survivors measured thicker bills than the five that died. (Similar patterns have been seen in much larger sample sizes during drought years on Daphne Major.)

Through their rapid transformation “the finches are redefining speciation,” Lamichhaney says. He notes that the universal theory of evolution hasn’t changed much in the last two centuries: It still leans on geographic isolation and the ability for organisms to interbreed and produce fertile offspring. But with the Big Birds, “the most important criteria is not whether they can breed with others; it’s if they do,” Lamichhaney says. What's more, because the Galapagos finches lack flashy feathers, they largely rely on song recognition to pick out their mates. The same goes for the Big Birds, which are more in tune with Española Cactus-Finches. And so, they've been pushed into a reproductive bubble, even while living side by side with Medium Ground-Finches.

The result? One of the only known examples of speciation triggered by hybrid vertebrates. While hybridization does occur among plants and animals—birds are especially susceptible because of their mobility—it rarely gives rise to a unique species. The Big Birds, however, are living proof that a mashup can evolve into something deeper.

This was all unexpected for Lamichhaney, who initially joined the Grants’ real-time experiment to find out how bill size and shape are determined on the molecular level. He answered that “million-dollar question” in 2015, when he pinpointed two linked genes in several species of Galapagos finches. A year later, he found key differences in the same regions of DNA between Big Bird hybrids and their parent species. Humans and fish have the “beak gene” pair, too, Lamichhaney says; though in them, it can lead to severe facial defects.

But there are further reasons why birds sport a vast collection of beaks. As the Grants and other scientists have shown, environmental catastrophes, limited eats, and competition between species can play a sizable role. For instance, Channel Island Song Sparrows have adapted to their dry California climate by growing bigger beaks, allowing them to shed heat without losing water. These external conditions help control how beak-specific genes operate, Maybellene Gamboa, a doctoral researcher at Colorado State University, says.

After doing tangential work with the Channel Island Song Sparrows, Gamboa is excited to see a fresh discovery emerge from the Galapagos. “The beak is so ecologically important and complex; we need to keep investigating its genomic architecture to find more compelling examples like Sangeet’s,” she says. Lucky for her, Lamichhaney is still hooked. In a few months he’s embarking on his first trip to Daphne Major, where he will join his old advisor Leif Andersson and the Grants to collect and decode DNA from the latest Big Bird generation. “People say Darwin’s finches represent the old and classic system—that there’s nothing new to study,” Lamichhaney says. “Well, I say I’ll be surprised if I’m not surprised again.”